The application refers to methods of forming a fiber coupling device. The application further refers to a fiber coupling device. Fiber coupling devices comprise a substrate having a substrate surface and at least one optoelectronic and/or photonic element. On the substrate surface and/or the at least one optoelectronic and/or photonic element, at least one fiber coupling alignment structure is arranged which is optically transmissive.
In this application, the term ‘fiber coupling device’ does not necessarily include the fiber or the fiber endpiece. Rather, a ‘fiber coupling device’, wherever addressed in the application, shall denote a device or arrangement sufficiently prepared to allow mounting of a fiber endpiece. Generally, fiber mounting can be done either directly after fabrication of the device or arrangement or at a later point in time, depending on the particular construction of the fiber coupling device.
Optical fibers used for transmitting signals by means of electromagnetic radiation with wavelengths in the visible or infrared range have to be coupled, that is properly mounted in a precisely aligned position, relative to optoelectronic or photonic elements of chips or other substrates in order to ensure proper signal transmission with sufficiently high coupling efficiency between the optical fibers and the chips or other substrates. Proper alignment is required for each fiber that is for both ends or endpieces of it, at or near the respective optoelectronic or photonic elements to be connected to, such as optical transmitters, receivers or transceivers. In the case of an optoelectronic rather than photonic element, the element may for instance be a light-detecting or light-emitting optoelectronic element, such as a VCSEL, a laser diode, a photodiode, a photodetector or any other element for emitting or detecting electromagnetic radiation.
Optoelectronic substrates, photonic substrates and photonically integrated substrates (PIC; photonically integrated chip) comprise one or a plurality of optoelectronic elements, or photonic elements, respectively. Optoelectronic substrates may for instance be (or include, arranged on a mounting substrate) semiconductor chips (made of silicon, indium phosphide or any other binary, ternary or quaternary semiconductor material and/or comprising plural layers of these and other materials, for instance), glass substrates, quartz substrates, ceramic substrates or synthetic substrates. Such optoelectronic and/or photonic substrates (hereinafter commonly referred to as ‘chips’ without further distinction) may be supported by additional mounting substrates, such as printed circuit boards (PCBs). Hereinafter, any arrangement of one or a plurality of chips, be it with or without a mounting substrate underneath, may generally be addressed when a ‘substrate’ is referred to herein below.
For coupling endpieces of optical fibers to photoelectronic or photonic elements, one or a plurality of fiber coupling alignment structures are required. In the simplest case, when a fiber endpiece is glued, by means of a light-transmissive glue droplet, to an optoelectronic element, precise alignment of the fiber endpiece to the active area of the optoelectronic element is required, which usually involves effort and some monitoring and adjustment of the fiber endpieces's position.
Conventionally, preshaped structures such as shrinked boot parts or preshaped parts molded by injection molding are used as fiber coupling alignment structures. These molding parts have to be mounted in proper alignment with respect to the optoelectronic or photonic elements of the chip or substrate. Any imprecise positioning of a molded fiber coupling alignment structure onto the respective substrate degrades the optical coupling efficiency (which can be actively measured as the percentage of light intensity actually transmitted through the fabricated chip-fiber-connection). Imprecise positioning of a molded fiber coupling alignment structure on the substrate thus reduces the tolerance for any positional mismatch between the fiber and the molded coupling structure itself during fiber assembly. Even if the ‘Fiber Coupling Alignment Structure’ (or ‘FCAS’ as subsequently referred to in the specification for the sake of brevity) and its contours (such as the fiber support surface, for instance a groove) were shaped very accurately, the fiber endpiece has still to be glued onto it. Although fiber mounting can be done by exploiting passive, self-aligned optical fiber coupling without the need to actively measure the amount of light intensity actually transmitted, any misalignment between the FCAS and the substrate narrows the tolerance margin left for mounting the fiber to the FCAS.
Many substrates, such as mounting substrates comprising at least one chip mounted thereon, comprise optoelectronic or photonic elements on their top main surface, thus resulting in a propagation direction of light to be emitted or detected which propagation direction is substantially normal to the active area or chip main surface or at least within an angular range of less than ±45°, such as less than ±20° from the normal direction of the active area or main surface. However, when the optical fiber approaches the chip substantially in parallel to its main surface, reflective mirror surfaces or other constructional elements or parts are required in order to reconcile the demands of optimum coupling efficiency and small vertical extension of the fiber-connected substrate. For instance, mirror surfaces with an orientation of about 45° (±5°) with respect to the main surface are often provided at or outside the fiber coupling alignment structure FCAS. If such additional reflecting elements are aligned improperly between the fiber endpiece and the substrate, signal transmission is degraded further. Even if the fiber coupling alignment structure is integrated in the FCAS, for instance as an inclined outer surface portion of it, any mismatch between the FCAS and the substrate causes a reflected light beam, after entering from the fiber, to be offset both in lateral as well as vertical directions, thus being less completely coupled to the optoelectronic or photonic element of the chip or substrate.
It is desirable to provide a method of forming a fiber coupling device and a fiber coupling device that ensure more precise mounting of an optical fiber in improved alignment and with even less manufacturing effort and costs.